This invention relates to devices for picking sheets from a stack of sheet media, and more particularly to a sheet feeding apparatus for picking and separating the top sheet in a stack.
Printers typically employ an input tray in which a stack of input print media such as sheets of paper or other print media are held. The printer has a sheet feeder which picks the top sheet of the stack and feeds the picked sheet along a paper path to a print area in which the printing operation takes place. In a known type of sheet feeder, during the pick operation, the stack is lifted up to a pick roller with a linearly stationary axis. The pick roller is then rotated to pick the top sheet from the stack and feed the picked sheet into the paper path. The consistency of the pick force with such an arrangement depends on the stack height as well as the amount of force delivered to lift the stack lip via a transmitted torque from a motor or spring. The torque margin and spring force inconsistencies provide challenges to overcome in order to achieve pick reliability. Moreover, this type of pick arrangement is limited to relatively short stacks of print media, since lifting higher media stacks, e.g. larger than 250 sheets, can be costly.
It would therefore be an advantage to provide a technique to pick sheets from a relatively large media stack, which provides a relatively constant pick force.
It would further be advantageous to provide a technique for picking the top sheet and also separating any multi-picked sheets.
A pick system is described for picking a top sheet of a medium from a media stack. The system includes a pick roller and a retard roller assembly arranged in cooperative relationship to form a nip between adjacent external surfaces of the pick roller and the retard roller. The pick roller is positioned adjacent a leading edge of the top sheet and in contact with a top surface of the top sheet. A pick roller drive apparatus is coupled to the pick roller for rotating the pick roller during a pick operation, the pick roller applying a force to the top sheet to drive the top sheet into the nip and into a media path. The retard roller includes a torque limiter apparatus for applying a retard force to the retard roller tending to resist the rotation of the retard roller as print media is driven into the nip during a pick operation. The retard force is less than a feed force applied to the top sheet by the pick roller as the top sheet is passed into the nip, the retard force being greater that a sheet-to-sheet frictional drag force between the top sheet and an adjacent sheet, thereby separating the adjacent sheet from the top sheet to prevent multiple sheet picks.
In accordance with an aspect of the invention, the pick system does not require a lifting structure to lift the stack during the pick operation. The pick force on the top sheet is relatively constant for various stack heights.
According to another aspect of the invention, a method is described for picking sheets from a stack of sheets, comprising the following steps:
positioning a pick roller and a retard roller assembly to form a nip between adjacent external surfaces of the pick roller and the retard roller, the pick roller being positioned adjacent a leading edge of the top sheet and in contact with a top surface of the top sheet;
rotating the pick roller during a pick operation to apply a force to the top sheet to drive the top sheet into the nip and into a media path; and
applying a retard force to the retard roller tending to resist the rotation of the retard roller as print media is driven into the nip during a pick operation, the retard force less than a feed force applied to the top sheet by the pick roller as the top sheet is passed into the nip, the retard force being greater than a sheet-to-sheet frictional drag force between the top sheet and an adjacent sheet, thereby separating an adjacent sheet from the top sheet to prevent multiple sheet picks.